Radiation, radicals, and images

Preserving Retinal Structure and Function with the Novel Nitroxide Antioxidant, DCTEIO

Oxidative stress is a major contributor to progressive neurodegenerative disease and may be a key target for the development of novel preventative and therapeutic strategies. Nitroxides have been successfully utilised to study changes in redox status (biological probes) and modulate radical-induced oxidative stress. This study investigates the efficacy of DCTEIO (5,6-dicarboxy-1,1,3,3-tetraethyllisoindolin-2-yloxyl), a stable, kinetically-persistent, nitroxide-based antioxidant, as a retinal neuroprotectant. The preservation of retinal function following an acute ischaemic/reperfusion (I/R) insult in the presence of DCTEIO was quantified by electroretinography (ERG). Inflammatory responses in retinal glia were analysed by GFAP and IBA-1 immunohistochemistry, and retinal integrity assessed by histology. A nitroxide probe combined with flow cytometry provided a rapid technique to assess oxidative stress and the mitigation offered by antioxidant compounds in cultured 661W photoreceptor cells. DCTEIO protected the retina from I/R-induced damage, maintaining retinal function. Histological analysis showed preservation of retinal integrity with reduced disruption and disorganisation of the inner and outer nuclear layers. I/R injury upregulated GFAP expression, indicative of retinal stress, which was significantly blunted by DCTEIO. The number of 'activated' microglia, particularly in the outer retina, in response to cellular stress was also significantly reduced by DCTEIO, potentially suggesting reduced inflammasome activation and cell death. DCTEIO mitigated oxidative stress in 661W retinal cell cultures, in a dose-dependent fashion. Together these findings demonstrate the potential of DCTEIO as a neuroprotective therapeutic for degenerative diseases of the CNS that involve an ROS-mediated component, including those of the retina e.g. age-related macular degeneration and glaucoma.

Metabolomics in aging research: aging markers from organs

Metabolism plays an important role in regulating aging at several levels, and metabolic reprogramming is the main driving force of aging. Due to the different metabolic needs of different tissues, the change trend of metabolites during aging in different organs and the influence of different levels of metabolites on organ function are also different, which makes the relationship between the change of metabolite level and aging more complex. However, not all of these changes lead to aging. The development of metabonomics research has opened a door for people to understand the overall changes in the metabolic level in the aging process of organisms. The omics-based "aging clock" of organisms has been established at the level of gene, protein and epigenetic modifications, but there is still no systematic summary at the level of metabolism. Here, we reviewed the relevant research published in the last decade on aging and organ metabolomic changes, discussed several metabolites with high repetition rate, and explained their role in vivo, hoping to find a group of metabolites that can be used as metabolic markers of aging. This information should provide valuable information for future diagnosis or clinical intervention of aging and age-related diseases.

Oxidative Desulfurization Activity of NIT Nitroxide Radical Modified Metallophthalocyanine

In the present study, metallophthalocyanines were modified with NIT nitroxide radicals through chemical bonds to prepare a series of metallophthalocyanines–NIT catalysts (MPcTcCl₈-NIT, M=Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) applied for oxidative desulfurization of thiophene (T) in model fuel. The MPcTcCl₈-NIT catalysts were characterized by FTIR, UV-Vis, ESR, and XPS spectra. The oxidative desulfurization activity of MPcTcCl₈-NIT catalysts was studied in a biomimetic catalytic system using molecular O₂ as the oxidant. The MPcTcCl₈-NIT catalysts exhibited high catalytic activities for the oxidation of thiophene in model fuel. The desulfurization rate of ZnPcTcCl₈-NIT for thiophene reached to 99.61%, which was 20.53% higher than that of pure ZnPcTcCl₈ (79.08%) under room temperature and natural light. The results demonstrated that MPcTcCl₈-NIT catalysts could achieve more effective desulfurization rate under milder conditions than that of the metallophthalocyanines. The NIT nitroxide radicals also could improve the catalytic activity of metallophthalocyanine based on the synergistic oxidation effect. The stability experiments for ZnPcTcCl₈-NIT showed that the catalyst still had a high desulfurization rate of 92.37% after five times recycling. All these findings indicate that the application of MPcTcCl₈-NIT catalysts provides a potential new way for the desulfurization performance of thiophene in fuel.

Estimation of the Local Concentration of the Markedly Dense Hydroxyl Radical Generation Induced by X-rays in Water

The linear-density (number of molecules on an arbitrary distance) of X-ray-induced markedly dense hydroxyl radicals (•OH) in water was estimated based on EPR spin-trapping measurement. A lower (0.13 mM-2.3 M) concentration series of DMPO water solutions and higher (1.7-6.0 M) concentration series of DMPO water solutions plus neat DMPO liquid (8.8 M as DMPO) were irradiated with 32 Gy of X-rays. Then, the yield of DMPO-OH in DMPO water solutions and the total spin-adduct of DMPO in neat DMPO were quantified. For the higher concentration DMPO series, the EPR peak area was estimated by double integration, and the baseline correction of the integral spectrum is necessary for accurate estimation of the peak area. The preparation of a suitable standard sample corresponding to the electric permittivity according to DMPO concentration was quite important for quantification of DMPO-OH, especially in DMPO concentration beyond 2 M. The linear-density of •OH generation in water by X-ray irradiation was estimated from the inflection point on the plot of the DMPO-OH yield versus DMPO linear-density. The linear-density of X-ray-induced markedly dense •OH was estimated as 1168 μm^-1, which was converted to 0.86 nm as the intermolecular distance and 2.6 M as the local concentration.

Nitroxyl Radical as a Theranostic Contrast Agent in Magnetic Resonance Redox Imaging

Significance:In vivo assessment of paramagnetic and diamagnetic conversions of nitroxyl radicals based on cyclic redox mechanism can be an index of tissue redox status. The redox mechanism of nitroxyl radicals, which enables their use as a normal tissue-selective radioprotector, is seen as being attractive on planning radiation therapy. Recent Advances:In vivo redox imaging using nitroxyl radicals as redox-sensitive contrast agents has been developed to assess tissue redox status. Chemical and biological behaviors depending on chemical structures of nitroxyl radical compounds have been understood in detail. Polymer types of nitroxyl radical contrast agents and/or nitroxyl radical-labeled drugs were designed for approaching theranostics. Critical Issues: Nitroxyl radicals as magnetic resonance imaging (MRI) contrast agents have several advantages compared with those used in electron paramagnetic resonance (EPR) imaging, while support by EPR spectroscopy is important to understand information from MRI. Redox-sensitive paramagnetic contrast agents having a medicinal benefit, that is, nitroxyl-labeled drug, have been developed and proposed. Future Directions: A development of suitable nitroxyl contrast agent for translational theranostic applications with high reaction specificity and low normal tissue toxicity is under progress. Nitroxyl radicals as redox-sensitive magnetic resonance contrast agents can be a useful tool to detect an abnormal tissue redox status such as disordered oxidative stress. Antioxid. Redox Signal. 36, 95-121.

Effects of LET on oxygen-dependent and-independent generation of hydrogen peroxide in water irradiated by carbon-ion beams

Linear energy transfer (LET) dependence of yields of O₂-dependent and O₂-independent hydrogen peroxide (H₂O₂) in water irradiated by ionizing radiation was investigated. The radiation-induced hydroxyl radical (•OH) generation in an aqueous solution was reported to occur in two different localization densities, the milli-molar (relatively sparse) and/or molar (markedly-dense) levels. In the milli-molar-level •OH generation atmosphere, •OH generated at a molecular distance of ∼7 nm are likely unable to interact. However, in the molar-level •OH generation atmosphere, several •OH were generated with a molecular distance of 1 nm or less, and two •OH can react to directly make H₂O₂. An aliquot of ultra-pure water was irradiated by 290-MeV/nucleon carbon-ion beams at the Heavy-Ion Medical Accelerator in Chiba (HIMAC, NIRS/QST, Chiba, Japan). Irradiation experiments were performed under aerobic or hypoxic (<0.5% oxygen) conditions, and several LET conditions (13, 20, 40, 60, 80, or >100 keV/μm). H₂O₂ generation in irradiated samples was estimated by three methods. The amount of H₂O₂ generated per dose was estimated and compared. O₂-independent H₂O₂ generation, i.e. H₂O₂ generation under hypoxic conditions, increased with increasing LET. On the other hand, the O₂-dependent H₂O₂ generation, i.e. subtraction of H₂O₂ generation under hypoxic conditions from H₂O₂ generation under aerobic conditions, decreased with increasing LET. This suggests that the markedly-dense •OH generation is positively correlated with LET. High-LET beams generate H₂O₂ in an oxygen-independent manner.

Radiation-Induced Senescence Reprograms Secretory and Metabolic Pathways in Colon Cancer HCT-116 Cells

Understanding the global metabolic changes during the senescence of tumor cells can have implications for developing effective anti-cancer treatment strategies. Ionizing radiation (IR) was used to induce senescence in a human colon cancer cell line HCT-116 to examine secretome and metabolome profiles. Control proliferating and senescent cancer cells (SCC) exhibited distinct morphological differences and expression of senescent markers. Enhanced secretion of pro-inflammatory chemokines and IL-1, anti-inflammatory IL-27, and TGF-β1 was observed in SCC. Significantly reduced levels of VEGF-A indicated anti-angiogenic activities of SCC. Elevated levels of tissue inhibitors of matrix metalloproteinases from SCC support the maintenance of the extracellular matrix. Adenylate and guanylate energy charge levels and redox components NAD and NADP and glutathione were maintained at near optimal levels indicating the viability of SCC. Significant accumulation of pyruvate, lactate, and suppression of the TCA cycle in SCC indicated aerobic glycolysis as the predominant energy source for SCC. Levels of several key amino acids decreased significantly, suggesting augmented utilization for protein synthesis and for use as intermediates for energy metabolism in SCC. These observations may provide a better understanding of cellular senescence basic mechanisms in tumor tissues and provide opportunities to improve cancer treatment.

Inhomogeneous generation of hydroxyl radicals in hydrogen peroxide solution induced by ultraviolet irradiation and in a Fenton reaction system

The density of hydroxyl radical (•OH) generation by degeneration of hydrogen peroxide (H₂O₂) during UVB irradiation and in a Fenton reaction system was estimated. The purpose of this study was to evaluate whether these reaction systems generate spatially uniform or inhomogeneous •OH from H₂O₂ in the reaction mixture. A series of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) solutions of several concentrations (0.13‒1661 mM) were prepared. For UVB irradiation, 1 μl of 98 mM, 980 mM, or 9.8 M H₂O₂ solution was added to a 100-μl aliquot of DMPO solution, and the reaction mixture was irradiated with UVB. For the Fenton reaction, 1 μl of 98 mM H₂O₂ and 1 μl of 100 mM FeSO₄ were added to a 100-μl aliquot of DMPO solution. After UVB irradiation or adding FeSO₄, the entire volume of the reaction mixture was drawn into PTFE tubing and measured by X-band EPR. The DMPO-OH concentration in the reaction mixture was plotted versus the molecular density of DMPO, and the density of •OH generation was estimated from an inflection point on the plotted profile. The local densities of the UV-induced •OH in the H₂O₂ water solutions depended on the concentration of H₂O₂ in the solution, and were likely localized. The energy absorption process of photons was suspected to occur in a step-wise manner in a limited volume. •OH generation in the Fenton reaction system was expected to be uniformly distributed, but inhomogeneous •OH generation was observed at the molecular level.

The protective effect of nitronyl nitroxide radical on peroxidation of A549 cell damaged by iron overload

Particulate pollution in the air has strong links with increased morbidity of cardiopulmonary diseases. Iron is one of the major carcinogens in air pollution and can produce hydroxyl radical which induce oxidative stress, lead to cell damage and even to cancer. In this work, a novel nitronyl nitroxide radical NITPh(OMe)₂ (2-(2,4-dimethoxyphenyl) -4,4,5,5- tetramethylimidazoline- 1- oxyl-3- oxide) was prepared and characterized by electron spin-resonance spectroscopy (ESR), X-ray crystal diffraction, Fourier transform infrared (IR), X-ray powder diffraction (XRD), elemental analysis, ultraviolet and visible spectra (UV-Vis), and the electronic transition processes was also calculated by time-dependent density functional theory (TDDFT) to analysis UV-Vis spectrum. In vitro cell model of oxidative damage was established by ferric ammonium citrate (FAC) overload, and NITPh(OMe)₂ was studied as a free radical scavenger to protect peroxidation of A549 cells. Results showed that NITPh(OMe)₂ could significantly alleviate the damage of A549 cells by iron overload in cell morphology, cell viability, cell proliferation and cell apoptosis. The apoptotic signaling pathway of A549 cells induced by FAC and the protection mechanism of NITPh(OMe)₂ were all discussed through the expression of three relating proteins, Bcl-2, Bax and DDIT3. This work confirms that nitroxide radicals are effective antioxidants, and have potential application in clinical practice as therapeutic agents.

Design and synthesis a mitochondria-targeted dihydronicotinamide as radioprotector

Radiation-induced damage to the mitochondrial macromolecules and electron transfer chain (ETC), causing the generation of primary and secondary reactive oxygen (ROS) species. The continuous ROS production after radiation will trigger cell oxidative stress and ROS-mediated nucleus apoptosis and autophagy signaling pathways. Scavenging radiation-induced ROS effectively can help mitochondria to maintain their physiological function and relief cells from oxidative stress. Nicotinamide is a critical endogenous antioxidant helping to neutralize ROS in vivo. In this study, we designed and synthetized a novel mitochondrial-targeted dihydronicotinamide (Mito-N) with the help of mitochondrial membrane potential to enter the mitochondria and scavenge ROS. According to experiment results, Mito-N significantly increased cell viability by 30.75% by neutralizing the accumulated ROS and resisting DNA strands breaks after irradiation. Furthermore, the mice survival rate also improved with the treatment of Mito-N, by effectively ameliorating the hematopoietic system infliction under lethal dose irradiation.

A quantitative analysis of carbon-ion beam-induced reactive oxygen species and redox reactions

The amounts of reactive oxygen species generated in aqueous samples by irradiation with X-ray or clinical carbon-ion beams were quantified. Hydroxyl radical (^•OH), hydrogen peroxide (H₂O₂), and the total amount of oxidation reactions, which occurred mainly because of ^•OH and/or hydroperoxy radicals (HO₂ ^•), were measured by electron paramagnetic resonance-based methods. ^•OH generation was expected to be localized on the track/range of the carbon-ion beam/X-ray, and mM and M levels of ^•OH generation were observed. Total ^•OH generation levels were identical at the same dose irrespective of whether X-ray or carbon-ion beam irradiation was used, and were around 0.28-0.35 µmol/L/Gy. However, sparse ^•OH generation levels decreased with increasing linear energy transfer, and were 0.17, 0.15, and 0.09 µmol/L/Gy for X-ray, 20 keV/µm carbon-ion beam, and >100 keV/µm carbon-ion beam sources, respectively. H₂O₂ generation was estimated as 0.26, 0.20, and 0.17 µmol/L/Gy, for X-ray, 20 keV/µm carbon-ion beam, and >100 keV/µm carbon-ion beam sources, respectively, whereas the ratios of H₂O₂ generation per oxygen consumption were 0.63, 0.51, and 3.40, respectively. The amounts of total oxidation reactions were 2.74, 1.17, and 0.66 µmol/L/Gy, respectively. The generation of reactive oxygen species was not uniform at the molecular level.

Brain Redox Imaging Using In Vivo Electron Paramagnetic Resonance Imaging and Nitroxide Imaging Probes

Reactive oxygen species (ROS) are produced by living organisms as a result of normal cellular metabolism. Under normal physiological conditions, oxidative damage is prevented by the regulation of ROS by the antioxidant network. However, increased ROS and decreased antioxidant defense may contribute to many brain disorders, such as stroke, Parkinson’s disease, and Alzheimer’s disease. Noninvasive assessment of brain redox status is necessary for monitoring the disease state and the oxidative damage. Continuous-wave electron paramagnetic resonance (CW-EPR) imaging using redox-sensitive imaging probes, such as nitroxides, is a powerful method for visualizing the redox status modulated by oxidative stress in vivo. For conventional CW-EPR imaging, however, poor signal-to-noise ratio, low acquisition efficiency, and lack of anatomic visualization limit its ability to achieve three-dimensional redox mapping of small rodent brains. In this review, we discuss the instrumentation and coregistration of EPR images to anatomical images and appropriate nitroxide imaging probes, all of which are needed for a sophisticated in vivo EPR imager for all rodents. Using new EPR imaging systems, site-specific distribution and kinetics of nitroxide imaging probes in rodent brains can be obtained more accurately, compared to previous EPR imaging systems. We also describe the redox imaging studies of animal models of brain disease using newly developed EPR imaging.

Synthesis and Radioprotective Activity of Mitochondria Targeted Dihydropyridines In Vitro

The radiation-induced damage to mitochondrial oxidative respiratory chain could lead to generating of superoxide anions (O²⁻) and secondary reactive oxygen species (ROS), which are the major resources of continuous ROS production after radiation. Scavenging radiation-induced ROS effectively can help mitochondria to maintain their physiological function and relief cells from oxidative stress. Dihydropyridines (DHPs) are biomimetic hydrogen sources that could protect cells against radiation damage. In this study, we designed and synthetized three novel mitochondrial-targeted dihydropyridines (Mito-DHPs) that utilize the mitochondrial membrane potential to enter the organelle and scavenge ROS. MitoTracker confirmed Mito-DHPs accumulation in mitochondria, and the DCFH-DA assay demonstrated effective ROS scavenging activity. In addition, the γ-H2AX and comet assay demonstrated the ability of Mito-DHPs to protect against both radiation and ROS-induced DNA strand breaks. Furthermore, Mito-DHP1 proved to be non-toxic and displayed significant radioprotection activity (p < 0.05) in vitro. Mito-DHPs are therefore promising antioxidants that could penetrate the membrane of mitochondria, scavenge excessive ROS, and protect cells against radiation-induced oxidative damage.

Synthesis and Characterization of a Rosmarinic Acid Derivative that Targets Mitochondria and Protects against Radiation-Induced Damage In Vitro

Mitochondrial dysfunction plays an important role in gamma-radiation-induced mediating oxidative stress. Scavenging radiation-induced reactive oxygen species (ROS) can help mitochondria to maintain their physiological function. Rosmarinic acid is a polyphenol antioxidant that can scavenge radiation-induced ROS, but the structure prevents it from accumulating in mitochondria. In this study, we designed and synthesized a novel rosmarinic acid derivative (Mito-RA) that could use the mitochondrial membrane potential to enter the organelle and scavenge ROS. The DCFH-DA assay revealed that Mito-RA was more effective than rosmarinic acid at scavenging ROS. DNA double-strand breaks, chromosomal aberration, micronucleus and comet assays demonstrated the ability of Mito-RA to protect against radiation-induced oxidative stress in vitro. These findings demonstrate the potential of Mito-RA as an antioxidant, which can penetrate mitochondria, scavenge ROS and protect cells against radiation-induced oxidative damage.

Radio-protective effect and mechanism of 4-Acetamido-2,2,6,6- tetramethylpiperidin-1-oxyl in HUVEC cells

OBJECTIVES

To search for more effective radiation protectors with minimal toxicity, a water-soluble nitroxides Acetamido-Tempol (AA-Tempol) was evaluated for potential radioprotective properties in HUVEC cells (Human Umbilical Vein Endothelial cell line).

METHODS

To study the anti-radiation effect of AA-Tempol in cell culture, the viability of irradiated HUVEC cells using a clonogenic survival assay was examined. The anti-apoptosis effects of AA-Tempol using Annexin V/propidium iodide staining in a flow cytometry assay was also evaluated. To elucidate the molecular mechanism of the anti-apoptosis effect of AA-Tempol against X-radiation induced HUVEC cell apoptosis, the expression of Bax, Bcl-2 and p53 and caspase-3 were examined. The changes in the level of malondialdehyde (MDA) and glutathione (GSH) in HUVEC cells after X-radiation were also investigated.

RESULTS

Pretreatment of the HUVEC cells colony with AA-Tempol 1 h before X-radiation significantly increased the colony survival (p < 0.05) compared with the cells without pretreatment. This demonstrates that AA-Tempol provides an effective radiation protection in the irradiated HUVEC cells, thus reducing apoptosis from 20.1 ± 1.3% in 8 Gy X-radiated cells to 12.2 ± 0.9% (1.0 mmol/L-1 AA-Tempol) in AA-Tempo pretreated HUVEC cells. This implies that 1.0 mM AA-Tempol treatment significantly block the increase of caspase-3 activity in radiated HUVEC cells (P < 0.01), causing down-regulation in expressions of Bax and P53 and up-regulation in the expression of Bcl-2. Pretreatment with AA-Tempol also decreased the MDA activities (P < 0.01) and increase the GSH level (P < 0.05) in HUVEC cells compared to the 8Gy X-radiated cells without pretreatment.

CONCLUSIONS

These observations indicate that AA-Tempol is a potential therapeutic agent against the radiation damage.

Generation of reactive oxygen species in the anterior eye segment. Synergistic codrugs of N-acetylcarnosine lubricant eye drops and mitochondria-targeted antioxidant act as a powerful therapeutic platform for the treatment of cataracts and primary open-angle glaucoma

Senile cataract is a clouding of the lens in the aging eye leading to a decrease in vision. Symptoms may include faded colors, blurry vision, halos around light, trouble with bright lights, and trouble seeing at night. This may result in trouble driving, reading, or recognizing faces. Cataracts are the cause of half of blindness and 33% of visual impairment worldwide. Cataracts result from the deposition of aggregated proteins in the eye lens and lens fiber cells plasma membrane damage which causes clouding of the lens, light scattering, and obstruction of vision. ROS induced damage in the lens cell may consist of oxidation of proteins, DNA damage and/or lipid peroxidation, all of which have been implicated in cataractogenesis. The inner eye pressure (also called intraocular pressure or IOP) rises because the correct amount of fluid can't drain out of the eye. With primary open-angle glaucoma, the entrances to the drainage canals are clear and should be working correctly. The clogging problem occurs further inside the drainage canals, similar to a clogged pipe below the drain in a sink. The excessive oxidative damage is a major factor of the ocular diseases because the mitochondrial respiratory chain in mitochondria of the vital cells is a significant source of the damaging reactive oxygen species superoxide and hydrogen peroxide. However, despite the clinical importance of mitochondrial oxidative damage, antioxidants have been of limited therapeutic success. This may be because the antioxidants are not selectively taken up by mitochondria, but instead are dispersed throughout the body, ocular tissues and fluids' moieties. This work is an attempt to integrate how mitochondrial reactive oxygen species (ROS) are altered in the aging eye, along with those protective and repair therapeutic systems believed to regulate ROS levels in ocular tissues and how damage to these systems contributes to age-onset eye disease and cataract formation. Mitochondria-targeted antioxidants might be used to effectively prevent ROS-induced oxidation of lipids and proteins in the inner mitochondrial membrane in vivo. The authors developed and patented the new ophthalmic compositions including N-acetylcarnosine acting as a prodrug of naturally targeted to mitochondria l-carnosine endowed with pluripotent antioxidant activities, combined with mitochondria-targeted rechargeable antioxidant (either MitoVit E, Mito Q or SkQs) as a potent medicine to treat ocular diseases. Such specificity is explained by the fact that developed compositions might be used to effectively prevent ROS-induced oxidation of lipids and proteins in the inner mitochondrial membrane in vivo and outside mitochondria in the cellular and tissue structures of the lens and eye compartments. Mitochondrial targeting of compounds with universal types of antioxidant activity represents a promising approach for treating a number of ROS-related ocular diseases of the aging eye and can be implicated in the management of cataracts and primary open-angle glaucoma.

Dynamic changes in the distribution and time course of blood-brain barrier-permeative nitroxides in the mouse head with EPR imaging: visualization of blood flow in a mouse model of ischemia

Electron paramagnetic resonance (EPR) imaging using nitroxides as redox-sensitive probes is a powerful, noninvasive method that can be used under various physiological conditions to visualize changes in redox status that result from oxidative damage. Two blood-brain barrier-permeative nitroxides, 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (HMP) and 3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine-1-yloxy (MCP), have been widely used as redox-sensitive probes in the brains of small animals, but their in vivo distribution and properties have not yet been analyzed in detail. In this study, a custom-made continuous-wave three-dimensional (3D) EPR imager was used to obtain 3D EPR images of mouse heads using MCP or HMP. This EPR imager made it possible to take 3D EPR images reconstructed from data from 181 projections acquired every 60s. Using this improved EPR imager and magnetic resonance imaging, the distribution and reduction time courses of HMP and MCP were examined in mouse heads. EPR images of living mice revealed that HMP and MCP have different distributions and different time courses for entering the brain. Based on the pharmacokinetics of the reduction reactions of HMP and MCP in the mouse head, the half-lives of HMP and MCP were clearly and accurately mapped pixel by pixel. An ischemic mouse model was prepared, and the half-life of MCP was mapped in the mouse head. Compared to the half-life in control mice, the half-life of MCP in the ischemic model mouse brain was significantly increased, suggesting a shift in the redox balance. This in vivo EPR imaging method using BBB-permeative MCP is a useful noninvasive method for assessing changes in the redox status in mouse brains under oxidative stress.

Biological importance of reactive oxygen species in relation to difficulties of treating pathologies involving oxidative stress by exogenous antioxidants

Findings about involvement of reactive oxygen species (ROS) not only in defense processes, but also in a number of pathologies, stimulated discussion about their role in etiopathogenesis of various diseases. Yet questions regarding the role of ROS in tissue injury, whether ROS may serve as a common cause of different disorders or whether their uncontrolled production is just a manifestation of the processes involved, remain unexplained. Dogmatically, increased ROS formation is considered to be responsible for development of the so-called free-radical diseases. The present review discusses importance of ROS in various biological processes, including origin of life, evolution, genome plasticity, maintaining homeostasis and organism protection. This may be a reason why no significant benefit was found when exogenous antioxidants were used to treat free-radical diseases, even though their causality was primarily attributed to ROS. Here, we postulate that ROS unlikely play a causal role in tissue damage, but may readily be involved in signaling processes and as such in mediating tissue healing rather than injuring. This concept is thus in a contradiction to traditional understanding of ROS as deleterious agents. Nonetheless, under conditions of failing autoregulation, ROS may attack integral cellular components, cause cell death and deteriorate the evolving injury.

Mitochondria as a target in treatment

Mitochondria are key organelles that perform essential cellular functions and play pivotal roles in cell death and survival signaling. Hence, they represent an attractive target for drugs to treat metabolic, degenerative, and hyperproliferative diseases. Targeting mitochondria with organelle-specific agents or prodrugs has proven to be an effective therapeutic strategy. More specifically, controlling the cellular ROS balance via selective delivery of an antioxidant "payload" into mitochondria is an elegant emerging therapeutic concept. Herein, we review the recent medicinal chemistry and clinical data of these exploratory strategies, which should point the way for future generations of therapeutics.

In vivo imaging of free radicals and oxygen

Free radicals are highly reactive compounds that play an essential role in many biological processes, both beneficial and deleterious. Detection and quantification of these species is critical to develop a better understanding of normal and pathophysiological functions at the cellular and tissue levels. Electron paramagnetic resonance (EPR) spectroscopy is the technique most commonly used for this purpose through the detection of exogenous probes or spin traps that interact with the free radical species of interest. Over the past several years, the spatial and temporal distribution of free radicals within cells and tissues has been of particular interest. This chapter briefly explains the principles and challenges in the use of EPR for biological samples and introduces the concept of EPR for free radical imaging purposes. In addition, specific examples are given for the use of EPR imaging in four principal areas: free radical probes, nitric oxide (NO), redox state, and oxygen (O(2)) concentration.

A mitochondria-targeted triphenylphosphonium-conjugated nitroxide functions as a radioprotector/mitigator

Removal of excessive mitochondrial reactive oxygen species by electron scavengers and antioxidants is a promising therapeutic strategy to reduce the detrimental effects of radiation exposure. Here we exploited triphenylphosphonium (TPP) cation as a means to target nitroxide radicals to mitochondria. We synthesized a library of TPP-conjugated nitroxides and tested their radioprotective effects in gamma-irradiated mouse embryo cells and human epithelial BEAS-2B cells. Cells were incubated with conjugates either before or after irradiation. We found that [2-(1-oxyl-2,2,6,6-tetramethyl-piperidin-4-ylimino)-ethyl]-triphenyl-phosphonium (TPEY-Tempo) significantly blocked radiation-induced apoptosis as revealed by externalization of phosphatidylserine on the cell surface and inhibition of cytochrome c release from mitochondria. Using electron paramagnetic resonance, we showed that TPEY-Tempo was integrated into cells and mitochondria, where it underwent one-electron reduction to hydroxylamine. TPEY-Tempo acted as an electron scavenger that prevented superoxide generation and cardiolipin oxidation in mitochondria. Finally, TPEY-Tempo increased the clonogenic survival rate of irradiated cells. The cellular integration efficiencies of nonradioprotective TPP conjugates, including Mito-Tempo (Alexis, San Diego, CA), were markedly lower, although these homologues were integrated into isolated succinate-energized mitochondria to a similar extent as TPEY-Tempo. We conclude that mitochondrial targeting of TPP-conjugated nitroxides represents a promising approach for the development of novel radioprotectors.

Mitochondrial targeting of electron scavenging antioxidants: Regulation of selective oxidation vs random chain reactions

Effective regulation of highly compartmentalized production of reactive oxygen species and peroxidation reactions in mitochondria requires targeting of small molecule antioxidants and antioxidant enzymes into the organelles. This review describes recently developed approaches to mitochondrial targeting of small biologically active molecules based on: (i) preferential accumulation in mitochondria because of their hydrophobicity and positive charge (hydrophobic cations), (ii) binding with high affinity to an intra-mitochondrial constituent, and (iii) metabolic conversions by specific mitochondrial enzymes to reveal an active entity. In addition, targeted delivery of antioxidant enzymes via expression of leader sequences directing the proteins into mitochondria is considered. Examples of successful antioxidant and anti-apoptotic protection based on the ability of targeted cargoes to inhibit cytochrome c-catalyzed peroxidation of a mitochondria-specific phospholipid cardiolipin, in vitro and in vivo are presented. Particular emphasis is placed on the employment of triphenylphosphonium- and hemi-gramicidin S-moieties as two effective vehicles for mitochondrial delivery of antioxidants.

Synthesis, characterisation, cytotoxicity and radioprotective effect of novel chiral nitronyl nitroxyl radicals

Nitroxyl compounds have been previously investigated as potential radioprotection drugs. To develop new radioprotectors, two kinds of novel chiral nitronyl nitroxyl radicals: L- tert-butyl 2-(4, 5-dihydro-4, 4, 5, 5-tetramethyl-3-oxido-1 H-imidazol-3-ium-1-oxyl-2-yl) pyrrolidine-1-carboxylate ( L-NNP) and L- tert-butyl 2-[(4-(4, 5-dihydro-4,4,5,5-tetramethyl-3-oxido-1 H-imidazol-3-ium-1-oxyl-2-yl)-2-methoxyphenoxy)methyl] pyrrolidine-1-carboxylate ( L-NNVP) have been synthesised. The cytotoxic and radioprotective effects of these two compounds were then evaluated in rat glioma C6 cells.

Assessment of tissue redox status using metabolic responsive contrast agents and magnetic resonance imaging

Regulation of tissue redox status is important to maintain normal physiological conditions in the living body. Disruption of redox homoeostasis may lead to oxidative stress and can induce many pathological conditions such as cancer, neurological disorders and ageing. Therefore, imaging of tissue redox status could have clinical applications. Redox imaging employing magnetic resonance imaging (MRI) with nitroxides as cell-permeable redox-sensitive contrast agents has been used for non-invasive monitoring of tissue redox status in animal models. The redox imaging applications of nitroxide electron paramagnetic resonance imaging (EPRI) and MRI are reviewed here, with a focus on application of tumour redox status monitoring. While particular emphasis has been placed on differences in the redox status in tumours compared to selected normal tissues, the technique possesses the potential to have broad applications to the study of other disease states, inflammatory processes and other circumstances where oxidative stress is implicated.

The imaging probe development center and the production of molecular imaging probes

The Imaging Probe Development Center (IPDC), part of the NIH Roadmap for Medical Research Initiative (http://nihroadmap.nih.gov/) recently became fully operational at its newly refurbished laboratories in Rockville, MD. The IPDC (http://nihroadmap.nih.gov/molecularlibraries/ipdc/) is dedicated to the production of known and novel molecular imaging probes, with its services currently being used by the NIH intramural community, although in the future it is intended that the extramural community will also benefit from the IPDC's resources. The Center has been set up with the belief that molecular imaging, and the probe chemistry that underpins it, will constitute key technologies going forward. As part of the larger molecular libraries and imaging initiative, it is planned that the IPDC will work closely with scientists from the molecular libraries effort. Probes produced at the IPDC include optical, radionuclide and magnetic resonance agents and may encompass any type of contrast agent. As IPDC is a trans-NIH resource it can serve each of the 27 Institutes and Centers that comprise NIH so its influence can be expected to impact widely different subjects and disease conditions spanning biological research. IPDC is expected to play a key part in interdisciplinary collaborative imaging projects and to support translational R&D from basic research through clinical development, for all of the imaging modalities. Examples of probes already prepared or under preparation are outlined to illustrate the breadth of the chemistries undertaken together with a reference outline of the diverse biological applications for which the various probes are intended.

A mitochondria-targeted nitroxide/hemigramicidin S conjugate protects mouse embryonic cells against gamma irradiation

PURPOSE

To evaluate the in vitro radioprotective effect of the mitochondria-targeted hemigramicidin S-conjugated 4-amino-2,2,6,6-tetramethyl-piperidine-N-oxyl (hemi-GS-TEMPO) 5-125 in gamma-irradiated mouse embryonic cells and adenovirus-12 SV40 hybrid virus transformed human bronchial epithelial cells BEAS-2B and explore the mechanisms involved in its radioprotective effect.

METHODS AND MATERIALS

Cells were incubated with 5-125 before (10 minutes) or after (1 hour) gamma-irradiation. Superoxide generation was determined by using dihydroethidium assay, and lipid oxidation was quantitated by using a fluorescence high-performance liquid chromatography-based Amplex Red assay. Apoptosis was characterized by evaluating the accumulation of cytochrome c in the cytosol and externalization of phosphatidylserine on the cell surface. Cell survival was measured by means of a clonogenic assay.

RESULTS

Treatment (before and after irradiation) of cells with 5-125 at low concentrations (5, 10, and 20 mum) effectively suppressed gamma-irradiation-induced superoxide generation, cardiolipin oxidation, and delayed irradiation-induced apoptosis, evaluated by using cytochrome c release and phosphatidylserine externalization. Importantly, treatment with 5-125 increased the clonogenic survival rate of gamma-irradiated cells. In addition, 5-125 enhanced and prolonged gamma-irradiation-induced G(2)/M phase arrest.

CONCLUSIONS

Radioprotection/mitigation by hemi-GS-TEMPO likely is caused by its ability to act as an electron scavenger and prevent superoxide generation, attenuate cardiolipin oxidation in mitochondria, and hence prevent the release of proapoptotic factors from mitochondria. Other mechanisms, including cell-cycle arrest at the G(2)/M phase, may contribute to the protection.

Targeting mitochondria

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are closely linked to degenerative diseases such as Alzheimer's disease, Parkinson's, neuronal death including ischemic and hemorrhagic stroke, acute and chronic degenerative cardiac myocyte death, and cancer. As a byproduct of oxidative phosphorylation, a steady stream of reactive species emerge from our cellular energy plants, the mitochondria. ROS and RNS potentially cause damage to all cellular components. Structure alteration, biomolecule fragmentation, and oxidation of side chains are trade-offs of cellular energy production. ROS and RNS escape results in the activation of cytosolic stress pathways, DNA damage, and the upregulation of JNK, p38, and p53. Incomplete scavenging of ROS and RNS particularly affects the mitochondrial lipid cardiolipin (CL), triggers the release of mitochondrial cytochrome c, and activates the intrinsic death pathway. Due to the active redox environment and the excess of NADH and ATP at the inner mitochondrial membrane, a broad range of agents including electron acceptors, electron donors, and hydride acceptors can be used to influence the biochemical pathways. The key to therapeutic value is to enrich selective redox modulators at the target sites. Our approach is based on conjugating nitroxides to segments of natural products with relatively high affinity for mitochondrial membranes. For example, a modified gramicidin S segment was successfully used for this purpose and proven to be effective in preventing superoxide production in cells and CL oxidation in mitochondria and in protecting cells against a range of pro-apoptotic triggers such as actinomycin D, radiation, and staurosporine. More importantly, these mitochondria-targeted nitroxide/gramicidin conjugates were able to protect against apoptosis in vivo by preventing CL oxidation induced by intestinal hemorrhagic shock. Optimization of nitroxide carriers could lead to a new generation of effective antiapoptotic agents acting at an early mitochondrial stage. Alternative chemistry-based approaches to targeting mitochondria include the use of proteins and peptides, as well as the attachment of payloads to lipophilic cationic compounds, sulfonylureas, anthracyclines, and other agents with proven or hypothetical affinities for mitochondria. Manganese superoxide dismutase (MnSOD), SS tetrapeptides with 2',6'-dimethyltyrosine (Dmt) residues, rhodamine, triphenylphosphonium salts, nonopioid analgesics, adriamycin, and diverse electron-rich aromatics and stilbenes were used to influence mitochondrial biochemistry and the biology of aging. Some general structural principles for effective therapeutic agents are now emerging. Among these are the presence of basic or positively charged functional groups, hydrophobic substructures, and, most promising for future selective strategies, classes of compounds that are actively shuttled into mitochondria, bind to mitochondria-specific proteins, or show preferential affinity to mitochondria-specific lipids.

Therapeutic and clinical applications of nitroxide compounds

Nitroxide compounds have been used for many years as biophysical tools, but only during the past 15-20 years have the many interesting biochemical interactions been discovered and harnessed for therapeutic applications. By modifying oxidative stress and altering the redox status of tissues, nitroxides have the ability to interact with and alter many metabolic processes. This interaction can be exploited for therapeutic and research use, including protection against ionizing radiation, as probes in functional magnetic resonance imaging, cancer prevention and treatment, control of hypertension and weight, and protection from damage resulting from ischemia/reperfusion injury. Although much remains to be done, many applications have been well studied, and some are presently being tested in clinical trials. The therapeutic and research uses of nitroxides are reviewed here, with a focus on the progress from initial development to modern, state-of-the art trials.

Reactivity of molecular oxygen with ethoxycarbonyl derivatives of tetrathiatriarylmethyl radicals

Tetrathiatriarylmethyl (TAM) radicals are commonly used as oximetry probes for electron paramagnetic resonance imaging applications. In this study, the electronic properties and the thermodynamic preferences for O2 addition to various TAM-type triarylmethyl (trityl) radicals were theoretically investigated. The radicals' stability in the presence of O2 and biological milieu was also experimentally assessed using EPR spectroscopy. Results show that H substitution on the aromatic ring affects the trityl radical's stability (tricarboxylate salt 1-CO2Na > triester 1-CO2Et > diester 2-CO2Et > monoester 3-CO2Et) and may lead to substitution reactions in cellular systems. We propose that this degradation process involves an arylperoxyl radical that can further decompose to alcohol or quinone products. This study demonstrates how computational chemistry can be used as a tool to rationalize radical stability in the redox environment of biological systems and aid in the future design of more biostable trityl radicals.

High-resolution mapping of tumor redox status by magnetic resonance imaging using nitroxides as redox-sensitive contrast agents

PURPOSE

There is considerable research directed toward the identification and development of functional contrast agents for medical imaging that superimpose tissue biochemical/molecular information with anatomical structures. Nitroxide radicals were identified as in vivo radioprotectors. Being paramagnetic, they can provide image contrast in magnetic resonance imaging (MRI) and electron paramagnetic resonance imaging (EPRI). The present study sought to determine the efficacy of nitroxide radioprotectors as functional image contrast agents.

EXPERIMENTAL DESIGN

Nitroxide radioprotectors, which act as contrast agents, were tested by EPRI and MRI to provide tissue redox status information noninvasively.

RESULTS

Phantom studies showed that the nitroxide, 3-carbamoyl-PROXYL (3CP), undergoes time-dependent reduction to the corresponding diamagnetic hydroxylamine only in the presence of reducing agents. The reduction rates of 3CP obtained by EPRI and MRI were in agreement suggesting the feasibility of using MRI to monitor nitroxide levels in tissues. The levels of 3CP were examined by EPRI and MRI for differences in reduction between muscle and tumor (squamous cell carcinoma) implanted in the hind leg of C3H mice simultaneously. In vivo experiments showed a T1-dependent image intensity enhancement afforded by 3CP which decreased in a time-dependent manner. Reduction of 3CP was found to be the dominant mechanism of contrast loss. The tumor regions exhibited a faster decay rate of the nitroxide compared to muscle (0.097 min(-1) versus 0.067 min(-1), respectively).

CONCLUSIONS

This study shows that MRI can be successfully used to co-register tissue redox status along with anatomic images, thus providing potentially valuable biochemical information from the region of interest.

Probing the Intracellular Redox Status of Tumors with Magnetic Resonance Imaging and Redox-Sensitive Contrast Agents

Nitroxide radicals are paramagnetic contrast agents, used in magnetic resonance imaging (MRI), that also exert antioxidant effects. Participating in cellular redox reactions, they lose their ability to provide contrast as a function of time after administration. In this study, the rate of contrast loss was correlated to the reducing power of the tissue or the "redox status." The preferential reduction of nitroxides in tumors compared with normal tissue was observed by MRI. The influence of the structure of the nitroxide on the reduction rate was investigated by MRI using two cell-permeable nitroxides, 4-hydroxy-2,2,6,6,-tetramethyl-1-piperidynyloxyl (Tempol) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (3CP), and one cell-impermeable nitroxide, 3-carboxy-2,2,5,5,5-tetramethylpyrrolidine-1-oxyl (3CxP). Pharmacokinetic images of these nitroxides in normal tissue, tumor, kidney, and artery regions in mice were simultaneously obtained using MRI. The decay of Tempol and 3CP in tumor tissue was significantly faster than in normal tissue. No significant change in the total nitroxide (oxidized + reduced forms) was noted from tissue extracts, suggesting that the loss in contrast as a function of time is a result of intracellular bioreduction. However, in the case of 3CxP (membrane impermeable), there was no difference in the reduction rates between normal and tumor tissue. The time course of T(1) enhancement by 3CxP and the total amount of 3CxP (oxidized + reduced) in the femoral region showed similar pharmacokinetics. These results show that the differential bioreduction of cell-permeable nitroxides in tumor and normal tissue is supported by intracellular processes and the reduction rates are a means by which the intracellular redox status can be assessed noninvasively.

Comparison of stable nitroxide, 3-substituted 2,2,5,5-tetramethylpyrrolidine-N-oxyls, with respect to protection from radiation, prevention of DNA damage, and distribution in mice

We compared three 3-substituted 2,2,5,5-tetramethylpyrrolidine-N-oxyls (PROXYLs): carbamoyl-, methoxycarbonyl-, and hydroxymethyl-PROXYL (CM-, MC-, and HM-PROXYL, respectively) with respect to radioprotection, prevention of DNA damage, and in vivo distribution in mice. The PROXYLs provided protection to C3H mice against lethal X-irradiation (8 Gy) with the following order of magnitude, HM- > CM- approximately MC-PROXYL. In contrast, radioprotection at the cellular level assessed by the colony formation of leukemia cell line L5178Y showed no difference among them. The degree of protection from X ray-induced oxidation of DNA bases measured by the formation of 8-hydroxydeoxyguanosine in salmon DNA and the cleavage of DNA measured by electrophoresis of plasmid pBR322 DNA did not differ among the PROXYLs. Redox potentials were also similar for each. However, the blood concentration of the PROXYLs injected ip into the mice showed different maximum concentrations (HM- > CM- approximately MC-PROXYL), although all reached a maximum at around 5-10 min and gradually decreased thereafter. Their concentration in bone marrow showed a similar pattern, suggesting that the difference in in vivo radioprotection among the three PROXYLs is due to the difference in their distribution to bone marrow. In general, the radioprotection provided by stable nitroxides is affected not only by redox potential and reactivity in vitro but also by pharmacokinetics.

Kinetics of tempol for prevention of xerostomia following head and neck irradiation in a mouse model

PURPOSE

Radiotherapy is commonly used to treat the majority of patients with head and neck cancers. Salivary glands in the radiation field are dramatically affected by this procedure. The purpose of this study was to examine pharmacokinetic characteristics of the stable nitroxide 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol) with respect to radioprotection of the salivary glands.

EXPERIMENTAL DESIGN

To evaluate the effect of different doses and times of administration, the heads of C3H mice were exposed to a single irradiation dose of 15 Gy, with i.p. tempol injection. To analyze other routes of administration, we injected 275 mg/kg tempol by an i.m., i.v., or s.c. route, 10 minutes before irradiation. We also tested whether oral administration of tempol in a topical form (either in a mouthwash or gel) provided any salivary gland protection.

RESULTS

Tempol treatment (137.5 or 275 mg/kg, i.p., 10 minutes before irradiation) significantly reduced irradiation-induced salivary hypofunction (approximately 50-60%). I.v. or s.c. administration of tempol also showed significant radioprotection, whereas i.m. administration proved to be ineffective. Topical use of tempol, either as a mouthwash or gel, also was radioprotective.

CONCLUSIONS

Our results strongly suggest that tempol is a promising candidate for clinical application to protect salivary glands in patients undergoing radiotherapy for head and neck cancers.

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Radioprotection by plant products: present status and future prospects

The development of radioprotective agents has been the subject of intense research in view of their potential for use within a radiation environment, such as space exploration, radiotherapy and even nuclear war. However, no ideal, safe synthetic radioprotectors are available to date, so the search for alternative sources, including plants, has been on going for several decades. In Ayurveda, the traditional Indian system of medicine, several plants have been used to treat free radical-mediated ailments and, therefore, it is logical to expect that such plants may also render some protection against radiation damage. A systematic screening approach can provide leads to identifying potential new candidate drugs from plant sources, for mitigation of radiation injury. This article reviews some of the most promising plants, and their bioactive principles, that are widely used in traditional systems of medicine, and which have rendered significant radioprotection in both in vitro and in vivo model systems. Plants and their constituents with pharmacological activities that may be relevant to amelioration of radiation-mediated damage, including antiemetic, antiinflammatory, antioxidant, cell proliferative, wound healing and haemopoietic stimulatories are also discussed.

Quantitative Measurements of Oxidative Stress in Mouse Skin Induced by X-Ray Irradiation

To find efficient methods to evaluate oxidative stress in mouse skin caused by X-ray irradiation, several markers and methodologies were examined. Hairless mice were irradiated with 50 Gy X-rays and skin homogenates or skin strips were prepared. Lipid peroxidation was measured using the skin homogenate as the level of thiobarbituric acid reactive substances. The level of lipid peroxidation increased with time after irradiation and was twice that of the control at 78 h. ESR spectra of skin strips showed a clear signal for the ascorbyl radical, which increased with time after irradiation in a manner similar to that of lipid peroxidation. To measure levels of glutathione (GSH) and its oxidized forms (GSSG) simultaneously, two HPLC methods, sample derivatization with 1-fluoro-2,4-dinitrobenzene and detection with a UV detector (method A) and no derivatization and detection with an electrochemical detector (method B), were compared and the latter was found to be better. No significant change was observed within 24 h after irradiation in the levels of GSH and GSSG measured by method B. The GSH/GSSG ratio may be a less sensitive parameter for the evaluation of acute oxidative stress caused by X-ray irradiation in the skin. Monitoring the ascorbyl radical seems to be a good way to evaluate oxidative stress in skin in vivo.

The effects of antioxidants on radiation-induced apoptosis pathways in TK6 cells

This study was designed to determine if radiation-mediated activation of the apoptotic pathways would be influenced by antioxidants and if a correlation would be found between radioprotection and changes in transduction pathways. Human lymphoblastoid TK6 cells, known to undergo apoptosis as a result of radiation, were irradiated (6 Gy) with and without antioxidants, and then whole-cell lysates were collected. Parallel studies were conducted to assess the survival (clonogenic assay) and apoptotic index. The impacts of two nitroxide antioxidants, tempol and CAT-1, differing in cell permeability, as well as the sulfhydryl antioxidant N-acetyl-L-cysteine (L-NAC), were estimated. Changes in apoptotic pathway proteins and p53 were assessed by Western blotting. Fraction of apoptotic cells was determined by flow cytometry. Tempol (10 mM), which readily enters cells, partially radioprotected TK6 cells against clonogenic killing, but had no effect on radiation-induced apoptotic parameters such as cleaved caspase 3 or cleaved PARP. Tempol alone did not induce cytotoxicity, yet did increase cleaved PARP levels. The radiation-induced increase in p53 protein was partly inhibited by tempol, but was unaffected by CAT-1 and L-NAC. Both CAT-1 (10 mM), which does not enter cells, and L-NAC (10 mM) had no radioprotective effect on cell survival. Although L-NAC did not protect against radiation-induced cytotoxicity, it completely inhibited radiation-induced increase in cleaved caspase 3 and cleaved PARP. Collectively, the results question the validity of using selected apoptosis pathway members as sole indicators of cytotoxicity.

The stable nitroxide tempol facilitates salivary gland protection during head and neck irradiation in a mouse model

PURPOSE

Radiotherapy is commonly used to treat a majority of patients with head and neck cancers. The long-term radiation-induced reduction of saliva output significantly contributes to the posttreatment morbidity experienced by these patients. The purpose of this study was to test the ability of the stable-free radical Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl), an established radioprotector, to prevent radiation-induced salivary hypofunction in mice.

EXPERIMENTAL DESIGN

The heads of C3H mice were exposed to a range of single radiation doses with or without an i.p. injection of 275 mg/kg Tempol 10 min before treatment. Salivary gland output was assessed 8 weeks postirradiation.

RESULTS

Radiation caused a dose-dependent reduction in salivary flow in this model. Tempol treatment alone significantly reduced radiation-induced salivary hypofunction. The combination of Tempol with mouth/nose shielding showed essentially complete radiation protection at 15 Gy and approximately 75% protection at 17.5 Gy.

CONCLUSIONS

This study demonstrates for the first time that significant radioprotection of the salivary glands is possible with Tempol in C3H mice.

Overexpression of extracellular superoxide dismutase protects mice from radiation-induced lung injury

PURPOSE

The purpose of this study was to determine if radiation-induced lung injury is associated with prolonged oxidative stress, and whether chronic overexpression of extracellular superoxide dismutase (EC-SOD) in the lung of transgenic mice protects against radiation-induced lung injury.

METHODS AND MATERIALS

Whole-lung radiation was delivered to EC-SOD overexpressing B6C3 transgenic (XRT-TG) mice and wild-type littermates (XRT-WT). Pulmonary function was assessed by breathing frequency. Right lung wet weight was used as a gross indicator of lung damage. Histopathology was used to assess collagen deposition and tissue fibrosis according to an established grading system. Immunohistochemistry was used to stain and quantify the number of macrophages. ELISA was used to measure activated TGF-beta1. Oxidative stress was assessed by measuring lipid oxidation products (malondialic acid) by HPLC.

RESULTS

Four of six XRT-WT mice required euthanasia at 15-19 weeks postradiation because of respiratory distress, whereas no XRT-TG mouse developed distress. All assessments of lung damage at 15-20 weeks postradiation were higher for XRT-WT mice compared with the XRT-TG mice, including breathing frequency (380 vs. 286 bpm, p <or= 0.0004), right lung weight (228 vs. 113 mg, p <or= 0.06), macrophage count (48 vs. 5 per 40x field, p <or= 0.06), and percent activated TGF-beta1 (37 vs. 11%, p <or= 0.06). Semiquantitative measures, including fibrosis and collagen deposition, were also higher for XRT-WT mice, with an exact Fisher p value of <or=0.03 for both variables. In addition, malondialic acid was elevated in XRT-WT mice 15-20 weeks after radiation delivery, and levels were lower in the XRT-TG mice (624 vs. 323 pmol/mg protein, p <or= 0.06).

CONCLUSIONS

After radiation therapy, oxidative stress is present at 15-20 weeks after initial exposure, which correlates with the delayed clinical onset of radiation-induced lung damage. Overexpression of EC-SOD in transgenic mice appears to confer protection against this radiation-induced lung injury, with a corresponding decrease in oxidative stress. EC-SOD may be a potential therapeutic agent for radioprotection in the treatment of thoracic malignancies. Further investigation is needed to confirm and expand on the current results.

Linking radiation oncology and imaging through molecular biology (or now that therapy and diagnosis have separated, it's time to get together again!)

Among the areas defined by the National Cancer Institute as "Extraordinary Opportunities for Research Investment" that are highly relevant to the technology-oriented disciplines within the broad field of radiology are cancer imaging, defining the signatures (ie, underlying molecular features) of cancer cells, and molecular targets of prevention and treatment. In molecular target credentialing, a specific molecular target is imaged, the molecular signature is defined, a treatment is given, and the effect of the intervention on the image findings and the signature is then evaluated. Such an approach is used to validate the proposed target as a legitimate one for cancer therapy or prevention and to provide the opportunity to ultimately individualize therapy on the basis of both the initial characteristics of the tumor and the tumor's response to an intervention. Therapeutic radiation is focused biology (ie, radiation produces molecular events in the irradiated tissue). Radiation can (a) kill cancer cells by itself, (b) be combined with cytotoxic or cytostatic drugs, and (c) serve to initiate radiation-inducible molecular targets that are amenable to treatment with drugs and/or biologic therapies. Focused biology can be anatomically confined with various types of external beams and with brachytherapy, and it can be used systemically with targeted radioisotopes. These new paradigms link diagnostic imaging, radiation therapy, and nuclear medicine in unique ways by way of basic biology. It is timely to develop new collaborative research, training, and education agendas by building on one another's expertise and adopting new fields of microtechnology, nanotechnology, and mathematical analysis and optimization.

Molecular and cellular biology of moderate-dose (1-10 Gy) radiation and potential mechanisms of radiation protection: report of a workshop at Bethesda, Maryland, December 17-18, 2001

Exposures to doses of radiation of 1-10 Gy, defined in this workshop as moderate-dose radiation, may occur during the course of radiation therapy or as the result of radiation accidents or nuclear/radiological terrorism alone or in conjunction with bioterrorism. The resulting radiation injuries would be due to a series of molecular, cellular, tissue and whole-animal processes. To address the status of research on these issues, a broad-based workshop was convened. The specific recommendations were: (1) RESEARCH: Identify the key molecular, cellular and tissue pathways that lead from the initial molecular lesions to immediate and delayed injury. The latter is a chronic progressive process for which postexposure treatment may be possible. (2) Technology: Develop high-throughput technology for studying gene, protein and other biochemical expression after radiation exposure, and cytogenetic markers of radiation exposure employing rapid and accurate techniques for analyzing multiple samples. (3) Treatment strategies: Identify additional biological targets and develop effective treatments for radiation injury. (4) Ensuring sufficient expertise: Recruit and train investigators from such fields as radiation biology, cancer biology, molecular biology, cellular biology and wound healing, and encourage collaboration on interdisciplinary research on the mechanisms and treatment of radiation injury. Communicate knowledge of the effects of radiation exposure to the general public and to investigators, policy makers and agencies involved in response to nuclear accidents/events and protection/treatment of the general public.

In vivo electron paramagnetic resonance oximetry with particulate materials

Electron paramagnetic resonance (EPR) methods can be used to study tissue pO(2) (PtO(2)) in anesthetized or awake animals (EPR oximetry). The method takes advantage of the fact that some paramagnetic materials have an EPR linewidth that is sensitive to the pO(2) in which the material is located. This article provides an overview of the method of EPR oximetry using implanted particulate materials as the sensors of pO(2). Characteristics of these materials are described to help the reader understand the factors involved in choosing the optimum particulate material. Examples of biological studies are included that show how EPR oximetry may be used on both awake and anesthetized animals.

Neuroprotection by the stable nitroxide 3-carbamoyl-proxyl during reperfusion in a rat model of transient focal ischemia

OBJECT

Nitroxides mimic superoxide dismutase (SOD) biochemically and may prevent free radical oxidative injury in settings in which endogenous SOD is overwhelmed. The authors have previously shown the efficacy of a nitroxide, Tempol, in reducing stroke infarct size. Of the nitroxides, 3-carbamoyl-proxyl (3-CP) is especially promising for clinical use, because it does not cause hypotension in animals. Its efficacy in brain ischemia, however, is untested. The goal of this study was to ascertain whether 3-CP would reduce brain damage in a rat ischemia-reperfusion model.

METHODS

The authors performed a blinded, dose-response study of the effect of different amounts of 3-CP (1, 10, and 100 mg/kg) on infarct size at 24 hours after focal ischemia and reperfusion. The 3-CP was given intravenously during reperfusion, which followed 1 hour of reversible ischemia induced by a thread placed intraluminally in the middle cerebral artery of rats. Brain infarcts, measured with 2,3,5-triphenyltetrazolium chloride staining in six 3-CP groups, were compared with those measured in controls (animals given an equal volume of saline). Edema-corrected infarct sizes (mean +/- standard deviation) were as follows: 146 +/- 64 mm3 in controls; 107 +/- 18 mm3 in rats given 1 mg/kg 3-CP; 40 +/- 20 mm3 in those given 10 mg/kg 3-CP; and 44 +/- 17 mm3 in those given 100 mg/kg 3-CP. A statistically significant reduction in infarct size was achieved in the 10- and 100-mg/kg 3-CP-treated groups (p < 0.01). A reduction in infarct size was also seen in the 1 mg/kg 3-CP-treated group, but this did not reach statistical significance. The authors observed no effects of 3-CP on blood pressure or brain temperature.

CONCLUSIONS

Given at reperfusion, 3-CP significantly decreases brain infarct size at doses of 10 and 100 mg/kg without causing hypotension. The authors found that 3-CP is well suited for further laboratory and clinical use in brain ischemia and reperfusion.

Differential protection by nitroxides and hydroxylamines to radiation-induced and metal ion-catalyzed oxidative damage

Modulation of radiation- and metal ion-catalyzed oxidative-induced damage using plasmid DNA, genomic DNA, and cell survival, by three nitroxides and their corresponding hydroxylamines, were examined. The antioxidant property of each compound was independently determined by reacting supercoiled DNA with copper II/1,10-phenanthroline complex fueled by the products of hypoxanthine/xanthine oxidase (HX/XO) and noting the protective effect as assessed by agarose gel electrophoresis. The nitroxides and their corresponding hydroxylamines protected approximately to the same degree (33-47% relaxed form) when compared to 76.7% relaxed form in the absence of protectors. Likewise, protection by both the nitroxide and corresponding hydroxylamine were observed for Chinese hamster V79 cells exposed to hydrogen peroxide. In contrast, when plasmid DNA damage was induced by ionizing radiation (100 Gy), only nitroxides (10 mM) provide protection (32.4-38.5% relaxed form) when compared to radiation alone or in the presence of hydroxylamines (10 mM) (79.8% relaxed form). Nitroxide protection was concentration dependent. Radiation cell survival studies and DNA double-strand break (DBS) assessment (pulse field electrophoresis) showed that only the nitroxide protected or prevented damage, respectively. Collectively, the results show that nitroxides and hydroxylamines protect equally against the damage mediated by oxidants generated by the metal ion-catalyzed Haber-Weiss reaction, but only nitroxides protect against radiation damage, suggesting that nitroxides may more readily react with intermediate radical species produced by radiation than hydroxylamines.

Radioprotection: the non-steroidal anti-inflammatory drugs (NSAIDs) and prostaglandins

Clinical and experimental studies of the acute and late effects of radiation on cells have enhanced our knowledge of radiotherapy and have led to the optimisation of radiation treatment schedules and to more precise modes of radiation delivery. However, as both normal and cancerous tissues have similar response to radiation exposure, radiation-induced injury on normal tissues may present either during, or after the completion of, the radiotherapy treatment. Studies on both NSAIDs and prostaglandins have indeed shown some evidence of radioprotection. Both have the potential to increase the survival of cells but by entirely different mechanisms. Studies of cell kinetics reveal that cells in the mitotic (M) and late G2 phases of the cell cycle are generally most sensitive to radiation compared with cells in the early S and G1/G0 phases. Furthermore, radiation leads to a mitotic delay in the cell cycle. Thus, chemical agents that either limit the proportion of cells in the M and G2 phases of the cell cycle or enhance rapid cell growth could in principle be exploited for their potential use as radioprotectors to normal tissue during irradiation. NSAIDs have been shown to exert anti-cancer effects by causing cell-cycle arrest, shifting cells towards a quiescence state (G0/G1). The same mechanism of action was observed in radioprotection of normal tissues. An increase in arachidonic acid concentrations after exposure to NSAIDs also leads to the production of an apoptosis-inducer ceramide. NSAIDs also elevate the level of superoxide dismutase in cells. Activation of heat shock proteins by NSAIDs increases cell survival by alteration of cytokine expression. A role for NSAIDs with respect to inhibition of cellular proliferation possibly by an anti-angiogenesis mechanism has also been suggested. Several in-vivo studies have provided evidence suggesting that NSAIDs may protect normal tissues from radiation injury. Prostaglandins do not regulate the cell cycle, but they do have a variety of effects on cell growth and differentiation. PGE(2) mediates angiogenesis, increasing the supply of oxygen and nutrients, essential for cellular survival and growth. Accordingly, PGE(2) at sufficiently high plasma concentrations enhances cellular survival by inhibiting pro-inflammatory cytokines such as TNF-alpha and IL-1beta. Thus, PGE(2) acts as a modulator, rather than a mediator, of inflammation. Prospective studies have suggested the potential use of misoprostol, a PGE(1) analogue, before irradiation, in prevention of radiation-induced side effects. The current understanding of the pharmacology of NSAIDs and prostaglandins shows great potential to minimise the adverse effects of radiotherapy on normal tissue.